Radio systems provide users of radio subscriber units with wireless communications. A particular type of radio system is a cellular radiotelephone system. A particular type of cellular radiotelephone system employs spread spectrum signalling. In such a system, a subscriber communication device such as a mobile station communicates with one or more remote base stations. Each base station provides communication in a fixed geographic area. As the mobile station moves among geographic areas, communication with the mobile station is handed off between the base stations.
Spread spectrum signalling can be broadly defined as a mechanism by which the bandwidth occupied by a transmitted signal is much greater than the bandwidth required by a baseband information signal. Two categories of spread spectrum communications are direct sequence spread spectrum (DSSS) and frequency-hopping spread spectrum (FHSS). The spectrum of a signal can be most easily spread by multiplying it with a wideband pseudorandom code-generated signal. It is essential that the spreading signal be precisely known so that the receiver can despread the signal. For DSSS, the objective of the receiver is to pick out the transmitted signal from a wide received bandwidth in which the signal is below the background noise level.
A cellular radiotelephone system using DSSS is commonly known as a Direct Sequence Code Division Multiple Access (DS-CDMA) system, according to Telecommunications Industry Association/Electronics Industries Association (TIA/EIA) interim standard IS-95. Individual users in the system use the same frequency but are separated by the use of individual spreading codes. Other spread spectrum systems include radiotelephone systems operating at 1900 MHz, as specified in American National Standards Institute (ANSI) standard J-STD-008. Other radio and radiotelephone systems use spread spectrum techniques as well.
In a spread spectrum communication system, downlink transmissions from a base station to a subscriber or mobile station include a pilot channel and a plurality of traffic channels. The pilot channel is decoded by all users. Each traffic channel is intended for decoding by a single user. Therefore, each traffic channel is encoded using a code known by both the base station and one mobile station. The pilot channel is encoded using a code known by the base station and all mobile stations.
In addition to the pilot channel and traffic channel signals, downlink transmissions also include a power control indicator in the traffic channel. The power control indicator is periodically transmitted by remote base stations to the mobile station to control the transmission power of the mobile station. The power control indicator conventionally includes several bits which are not encoded in any way. The power control indicator is binary in nature, in that it either tells the mobile station to increase transmit power or decrease transmit power. In response to the power control indicator, the mobile station adjusts its transmission power to accommodate changing channel conditions, such as fading or blocking or the sudden absence of these. For accurate, reliable communication, rapid response by the mobile station to the received power control indicator is necessary.
Mobile stations for use in spread spectrum communication systems commonly employ rake receivers. A rake receiver includes two or more receiver fingers which independently receive radio frequency (RF) signals. Each finger estimates channel gain and phase and demodulates the RF signals to produce traffic symbols. The traffic symbols of the receiver fingers are combined in a symbol combiner to produce a received signal.
Generally, the rake receiver fingers are assigned to the strongest channel multipath rays. That is, a first finger is assigned to receive the strongest signal, a second finger is assigned to receive the next strongest signal, and so on. As received signal strength changes, due to fading and other causes, the finger assignments are changed. Also, when the mobile is in a condition known as soft handoff, the fingers may be assigned to any of the base stations involved in the handoff. In soft handoff, the mobile station and base stations determine which base station provides optimum communication with the mobile station.
An average measure of multipath strength is employed to determine if a finger should be reassigned. The measure of multipath strength is the received signal-to-interference ratio (RSSI), also referred to as a received signal strength indication. The RSSI measurement is compared to predetermined lock and unlock thresholds. If the RSSI for a given finger is greater than the lock threshold, the finger is said to be locked. If the RSSI value is less than the unlock threshold, the finger is unlocked. The RSSI circuit provides a lock indication to a controller which controls the lock status of the individual fingers.
Good power control performance requires a fast RSSI circuit. The RSSI circuit should track Rayleigh fading and unlock a finger if a received signal momentarily drops into a fade. A weak link whose power control bits are demodulated incorrectly can cause the mobile station to respond incorrectly to the power control indication. This can cause dropped calls and other undesirable conditions. Thus, for power control bit decoding, it is necessary that the RSSI circuit be fast enough to unlock any finger that drops into a deep fade for longer than, for example, 10 ms.
However, the rapid response to fading required for power control bit decoding is not required for traffic channel demodulation. Accordingly, there is a need in the art for a rake receiver circuit and method in which a fast RSSI circuit provides a lock indication for power control bit decoding and a slow RSSI circuit provides a separate lock indication for traffic channel decoding. This would allow performance to be tailored to the individual requirements of the traffic channel decoding and the power control channel decoding. This, in turn, allows the performance to be optimized for accurate demodulation of both power control bits and traffic bits. The RSSI circuit is time-shared between the traffic channel and power control channel to minimize hardware requirements.